Intraluminal endovascular stenting is a method by which a prosthesis is inserted into a body tube and expanded so as to reopen a collapsed vessel wall and prevent the wall from recollapsing into the lumen. Endovascular stenting is particularly useful for arteries which are blocked or narrowed and is an alternative to surgical procedures that intend to bypass the occlusion.
Previous structures used as stents or intraluminal vascular grafts have included coiled stainless steel springs; helical wound spring coil made from shape memory alloy; expanding metal stents formed in a zig-zag pattern; diamond shaped, rectangular shaped, and other mesh and non-mesh designs. Some of the stents currently available employ a self expanding concept, whereby stent expansion is primarily achieved by removing a restraint mechanism holding the stent in a constricted configuration. Other stents in the prior art are delivered to the site by a balloon catheter system, and primarily employ balloon dilation to achieve proper stent expansion
Problems with this variety of stents is inadequate radial force to maintain expansion; inappropriate scaffolding of tissue to the wall; pre-dilated longitudinal rigidity which negatively impacts on stent delivery; and shortening of the stent as a consequence of radial expansion. Pre-dilation stent longitudinal rigidity is a significant shortcoming, and prevents the threading of the stent through long tortuous vessels and lesions. Shortening of the stent is also a problem, as it is important that the stent cover the entire lesion to minimize the risk of post-operative complications. Obviously, therefore, there is a need for a long yet flexible stent that will provide the appropriate scaffolding effect, will be able to track well in-curved vessels, will not shorten during radial expansion, and yet will have sufficient outward radial force to hold the artery open, even in the presence of hard calcified lesions. The stent disclosed herein overcomes these disadvantages. No stent having all of the desired features appears to exist, prior to this invention which achieves most of these properties.
As is well known, a traditional alternative to conventional vascular surgery has been percutaneous transluminal balloon angioplasty (PTCA). In this procedure, the angioplasty balloon is inflated within the stenosed artery to create, through shearing and mechanical trauma, a larger inner lumen. This process, while successful in achieving a larger lumen in most cases, can sometimes cause local tears, dissections and protrusion of plate into the lumen so that vessel blockage is caused rather than the desired vessel opening. In addition, the phenomenon of elastic recoil and intimal growth following arterial dilation often causes late restenosis (within six months) in more than about 30% of the patients undergoing the angioplasty balloon procedure. Because of the fear of the acute complication of sudden occlusion (abrupt closure), a surgical backup is needed in most places where PTCA is performed, This is yet another limitation of the mechanical balloon dilatation procedure.
It has been shown that stenting results in excellent acute results with adequate scaffolding of tears to the wall of the artery and with generation of a large inner lumen. This large inner lumen, initially present after stenting, has a lower restenosis rate after the procedure, as shown in the STRESS (N. Engl. J. Med. 1994; 331:L 496-501) and BENESTENT (N. Engl. J. Med. 1994; 331: 489-95) studies. While the inner lumen achieved using the self-expanding stents depends on the sizing of the stents relative to the vessel, the inner lumens that can be achieved with balloon expandable stents depend both on the size and radially expanding pressure of the balloon. The inner lumens achievable with balloon expandable stents can be further increased with further inflation of the balloon.
One of the major complications associated with stent use has been thrombosis. The problem occurs most commonly between day 2 and 6 of the implantation, but may also occur as late as 3 weeks after stenting. This complication is caused by clotting of the stent and is associated with high morbidity and mortality. It has been recently shown that the better the stent apposition against the wall and the larger the lumen is, the less likely that this complication will occur. In addition, it is very important that the stent cover the entire lesion since the existence of obstructions before or after the stent may also cause a complication.
The current balloon expandable stents have the significant limitation of relative, longitudinal rigidity during delivery, and so do not allow for a very long stent to traverse the usual curves in the artery. This longitudinal rigidity during delivery is sought to be avoided by devices taught in the patents to Wolff (U.S. Pat. No. 5,104,404) and to Pinchasik (U.S. Pat. No. 5,449,373) in which the rigid Palmaz stent sections are connected together with flexible connections. For this reason, it is required that the stent be long (to allow treatment of long lesions) and flexible upon insertion to site (to allow passage to and through tortuous locations) but yet have large radial force to unblock the vessel and excellent scaffolding so as to be able to hold the atherosclerotic material against the wall, even in bends and in hard calcified lesions. The stent should also allow for further balloon expansion if further lumen enlargement is required at particular locations.
In U.S. Pat. No. 5,104,404, Wolff attempts to address some of the shortcomings of the prior art by teaching the use of different connectors (articulation) between the rigid Palmaz stent segments, enabling more flexibility between the rigid parts.
It would be highly desirable, however, to have a stent having few or no longitudinally rigid parts so that it will be homogeneously flexible along its entire longitudinal axis when delivered on the catheter. Furthermore it would be extremely desirable to eliminate the longitudinal shortening of the stent during radial expansion to minimize stent misplacement.
Furthermore, in Palmas' stents marketed by Johnson & Johnson, as well as in others, during plastic deformation of the stent (i.e. balloon expansion) the strain is concentrated at small zones. This limits the properties of the material that can be used as well as the radial force and the expansion rate. By distributing the strain over large zones, a less thick annealed material can be used to both avoid deterioration of the radial force of the stent when expanded, and to reduce the stent's constricted profile. There are obvious advantages to reduced stent thickness.
According to the prior art method of manufacturing stents, the material is originally flat. The screen-like material is then rolled into a cylinder shape and laser welded or otherwise connected to form a tube--the weld running the length of the longitudinal axis. This is a difficult and expensive manufacturing procedure. It also leads to a potential lack of uniformity. The present invention, a new method of stent manufacture, as will be explained, results in a more uniformly expandable stent, one not having a weld line formed after mesh formation.
Patents which relate to the field of stent geometry are as follows: U.S. Pat. Nos. 5,354,309; 4,776,337; 5,356,423; 5,383,892; 5,178,618; 5,449,373; and 5,104,404.